Frequency hopping spread spectrum communication in mesh networks

ABSTRACT

A mesh network comprises a controller and a plurality of mesh-networked devices, operable to communicate with the controller. The controller and the plurality of mesh-networked devices comprise timing units, and are operable to communicate in accordance with a frequency hopping sequence. The mesh-networked devices switch between a transmit mode, in which they are capable of transmitting messages to one or more other mesh-networked device and/or the controller and an inactive mode, in which they are unable to transmit to or receive messages from one or more other mesh-networked devices or the controller. The timing units of the mesh-networked devices are synchronised and the mesh-networked devices are operable to switch to the transmit mode at pseudorandom time intervals.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a national phase entry under 35 U.S.C. § 371of International Application No. PCT/GB2017/053818, filed Dec. 19, 2017,which designates the United States of America, which claims priority toGB Application No. 1621881.0, filed Dec. 21, 2016, the entiredisclosures of each of these applications are hereby incorporated byreference in their entireties and for all purposes.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to frequency hopping spread spectrum(FHSS) communication in mesh networks. In particular the inventionrelates to a method of synchronising mesh networks to use FHSScommunication and especially to a method of synchronising mesh networksto use FHSS communication in battery powered and/or wireless meshnetworks, such as alarm systems.

BACKGROUND TO THE INVENTION

Point to point wireless communications are inherently unreliable.Atmospheric conditions, obstructing media and external interference areall subject to change which can attenuate or inhibit the wirelesscommunication path. A proven way to mitigate these factors is to form amesh network where alternative paths are provided for any device shouldthe primary path fails.

The main drawback of such an approach is all devices must be capable ofreceiving messages as well as transmitting so the current consumptioncan increase. To reduce this effect, a method of time synchronising bothtransmission and reception so that the time spent in the active statesis kept to a minimum is set out in WO2011/009646 (Texecom).

In a true battery powered mesh network, all nodes forward (i.e. receiveand send on) messages, rather than only locally powered nodes forwardingmessages, and battery powered nodes playing no part in the forwarding ofmessages. In this case, it is necessary for all nodes to wakeupperiodically to check for a transmitter preamble. In order to keep powerusage as low as possible, this need to happen very quickly; if there isno preamble to receive, the receiver must be shut off as quickly aspossible to conserve battery life.

In the system of WO2011/009646, a single hub or “control device”communicated with a number of security devices, the security devices aresynchronised to switch to the activated state for typically 18 ms onceevery 0.5 seconds and to remain in the activated state to communicatewith each other and transfer data.

Frequency Hopping Spread Spectrum (FHSS) is a method of radiocommunication involving switching the carrier through many frequencychannels in a sequence known to both the transmitter and receiver.

The main advantage of this is a much better resistance to narrowbandinterference than single channel systems. If one of the channel is beingused or blocked, the FHSS system can hop to the next channel. FHSSsystems are also very good at sharing bandwidth with other systems anddo not adding significant noise to channels not being used.

The dwell time in any channel ranges from a few bits within a packet(microseconds) to several hundred milliseconds. Some systems transmitshort packets occupying a single channel while others transmit longerpackets which are spread over many channels.

Approvals bodies worldwide, including FCC in the USA, often allow higherpower transmissions in systems that employ FHSS using a high number of(over 50) channels.

One challenge associated with any FHSS system is the synchronisation ofthe transmitter and the receiver.

One way of achieving this is to have a guarantee that the transmitterwill use all the channels during the preamble in a fixed period of time.The receiver can then find the transmitter by picking a random channeland listening for the preamble on that channel. While this works, itrequires both a long preamble from the transmitter and also the receiverto be active for a significant period of time in order to guaranteehearing the transmitter.

Alternatively, the receiver can cycle through the channels in order topick out a transmitter preamble on a single channel but this is subjectto the same drawback.

Neither of these methods of FHSS synchronisation would be suitable in abattery powered mesh network. In a 50 channel system, for example, eachreceiver would need to spend 50 times longer in receive each time it wassearching for a preamble which would have a catastrophic effect onbattery life.

In consequence, FHSS has been considered inappropriate for mesh networkswith battery powered nodes, and indeed even for locally powered nodes,if low-power operation is desirable.

FHSS can be useful in situations where security is important, becausedata is sent over different channels, so listening/blocking devicescannot simply block the one channel on which data is transferred (unlikesingle-channel systems). However, especially where a FHSS system movesconsecutively through channels, such listening/blocking devices candetermine the hopping sequence and block/listen to the correct channelby switching channels each time data ceases to be received on thechannel which is being listened to.

This could be overcome by switching between channels to a pseudorandomnew channel. Pseudorandom sequences satisfy one or more statisticaltests for randomness but are in fact produced by a definite mathematicalprocedure. It can be difficult to set up networked devices to follow thesame pseudorandom sequence, especially when new devices join the networkat different times in the sequence.

This invention aims to obviate, ameliorate, or mitigate one or more ofthe aforesaid problems of high power use when listening for a messageand difficulty in providing a secure network, and/or to provide improvedmesh networks/methods/devices.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a meshnetwork comprising a controller and a plurality of mesh-networkeddevices, operable to communicate with the controller; the controller andthe plurality of mesh-networked devices comprising timing units, andbeing operable to communicate in accordance with a frequency hoppingsequence; wherein the mesh-networked devices are operable to switchbetween a transmit mode, in which they are capable of transmittingmessages to one or more other mesh-networked device and/or thecontroller and an inactive mode, in which they are unable to transmit toor receive messages from one or more other mesh-networked devices or thecontroller; wherein the timing units of the mesh-networked devices aresynchronised and the mesh-networked devices are operable to switch tothe transmit mode at time intervals having a pseudorandom component.

Time intervals having a pseudorandom component are described hereinafteras “pseudorandom time intervals” and may be made up of a fixed component(i.e. a period of fixed length) and a pseudorandom component (i.e. aperiod having a variable length that satisfied one or more statisticaltest for randomness, but is in fact produced by a definite mathematicalprocedure), which is added to (or deducted from) the fixed component.Alternatively, the pseudorandom time intervals may be entirely composedof a pseudorandom component, but set between certain threshold values.

FHSS systems are more difficult to intercept, especially in real-time,than single channel systems, and by switching states at pseudorandomtime intervals, and switching between channels in accordance with thefrequency hopping sequence, it is even more difficult for a hacker tointercept the signals, and especially hard for a hacker to send falsesignals, e.g. to disarm a system, since the pseudorandom nature of thetime of switching makes it unknown to the hacker when the nexttransmission/reception will be, and unknown which channel it will be in.

The mesh-networked devices may be operable to switch between thetransmit mode, the inactive mode and a receive mode, in which they arecapable of receiving messages from one or more other mesh-networkeddevice and/or the controller. The mesh networked devices may be operableto switch to the receive mode in accordance with the frequency hoppingsequence.

The pseudorandom time intervals may be, on average, at least 30 seconds,at least one minute, at least 2 minutes, at least 10 minutes or at least15 minutes.

The pseudorandom time intervals may have minimum threshold values andmaximum threshold values. The average pseudorandom time interval may bethe mean of the minimum threshold value and the maximum threshold value.

The pseudorandom time intervals may have a fixed time component and apseudorandom time component (which could be positive or negative). Theaverage time interval may be the average pseudorandom time componentplus the fixed time component. If the pseudorandom time component isgenerated so as to be pseudorandomly positive or negative, over time itwill increase or decrease the fixed value by about the same amount, so,on average, the pseudorandom time interval will be equal to the fixedtime component.

The pseudorandom time intervals may be synchronised to occur when othermesh network devices are in the receive mode.

The mesh-networked devices may switch to the receive mode each time theychange frequency channel.

The time in each channel in the frequency hopping sequence, or “channeltime”, may be identical, or the channel time may be different fordifferent channels. Spending an identical time in each channel can bethe best use of resources, as the maximum time period can be spent ineach channel, and over time achieves an even spread across all thechannels. On the other hand, spending different amounts of time in eachchannel can make it more difficult for a hacker to determine thesequence, even if the sequence is cyclic. The channel time may be nomore than 1 second, no more than 0.5 seconds, or no more than 0.25seconds.

The channel time may be substantially shorter than the averagepseudorandom time interval, for example at least 10 times shorter, atleast 50 times shorter, at least 100 times shorter, or at least 200times shorter.

This arrangement, whereby the timing units are synchronised, and afrequency hopping sequence is followed means that no extra time needs tobe spent synchronising the devices in the frequency domain, as they arealready synchronised in the time domain.

The plurality of mesh-networked devices may be operable to switch to thereceive mode and remain in a receive mode for a first predetermined“wake-up” period, and if no message is received within that period, toreturn to the inactive mode.

The first predetermined “wake-up” period may be short, e.g. less than0.1 s, less than 0.05 s, or less than 0.02 s, less than 1 ms, less than0.5 ms, or less than 0.25 ms.

The plurality of mesh-networked devices may be operable to switch to thereceive mode and remain in a receive mode for a first predetermined“wake-up” period, and if a message is received within that period, toremain in an active (receive, or transmit) mode for a secondpredetermined “message forwarding” period.

The second predetermined “message forwarding” period may be longer thanthe first predetermined wake-up period. The second predetermined messageforwarding period may be sufficiently long to receive and then forward amessage. The plurality of mesh-networked devices may be operable toforward messages where the messages contain instructions to do so. Thesecond predetermined “message forwarding” period may be sufficientlylong to receive and forward a message to another device, and to receiveand forward an acknowledgement from said other device. The plurality ofmesh-networked devices may be operable to receive and forward a messageto another device, and to receive and forward an acknowledgement fromsaid other device during the second predetermined “message forwarding”period.

The second predetermined “message forwarding” period may be less than0.5 s, less than 0.2 s, or less than 0.05 s. The first predetermined“wake-up” period and the second predetermined “message forwarding”period may, in total be no more than 1 second, or no more than 0.5seconds.

This arrangement (in which the timing units of the devices aresynchronised, and they follow a frequency hopping sequence) allowsmesh-networked devices to wake up, listen briefly on the channel onwhich any transmissions would be received, and if no such transmissionis received, to return to the low-power inactive mode. The fact that themesh-networked devices are operable to communicate in accordance with afrequency hopping sequence, and switch to the transmit mode according toa pseudorandom time sequence, means that the use of the channels ispseudorandom

The frequency hopping sequence may be cyclic, the frequency hoppingsequence may be cyclic and consecutive, hopping to the next consecutivechannel (up or down) after a predetermined time interval.

Alternatively, the frequency hopping sequence may be cyclic, but notconsecutive, not necessarily switching from one channel to the nextfrequency channel up or down, but always following the same sequencefrom one channel to the next predetermined channel (e.g. always movingto channel 4 from channel 1).

The cycle of the frequency hopping sequence may be shorter than thepseudorandom component of the pseudorandom time interval.

The frequency hopping sequence could itself be pseudorandom (in whichcase each mesh-network device would have to follow the same pseudorandomsequence at the same time).

A pseudorandom frequency hopping sequence makes it very difficult for ahacker, but it also makes it difficult for devices to synchronise.Following a cyclic sequence and relying on the pseudorandom timeintervals makes synchronisation more straightforward.

The plurality of mesh-networked devices may follow the same frequencyhopping sequence at the same time.

All the mesh-networked devices in the mesh network may follow the samefrequency hopping sequence at the same time.

The controller may comprise a master clock and the mesh-networkeddevices may synchronise their timing units to the master clock.

The mesh-networked devices and optionally the controller, may beoperable to send acknowledgement messages comprising the currentfrequency channel and time left in the channel. This aidssynchronisation, since once the channel and the time left in the channelis known, a device knows where it is in the cycle through the channels.

The mesh-networked devices may be operable to receive acknowledgementmessages comprising the current frequency channel of a sending deviceand the time left in that channel and may compare that to their currentchannel to detect lack of synchrony with the sending device. The meshnetworked devices may be operable to correct their clock and/or thetiming of their frequency hopping based on the comparison with data inthe acknowledgement message.

The mesh-networked devices may achieve the switching to the transmitmode at pseudorandom time intervals by switching to the transmit modeaccording to a pseudorandom time sequence.

The mesh-networked devices may all switch to the transmit mode accordingto the same pseudorandom time sequence.

The mesh-networked devices may additionally switch to the transmit modewhen instructed to forward a message by another mesh-networked device,or the controller. This in effect is still pseudorandom, since theinstructions to forward a message will be based on a pseudorandomtransmission from the other mesh networked device or the controller.

The mesh networked devices may be battery powered.

The mesh network may be an alarm network.

The mesh networked devices may comprise sensors.

The mesh networked devices and optionally the controller may be operableto only send data packets short enough to fit within a single channel,e.g. less than 0.5 s, less than 0.1 s, or less than 50 ms, when they aresynchronised. Devices that are not yet synchronised to the mesh network,may send longer data packets, in order to assist joining the meshnetwork.

The plurality of mesh-networked devices, may be operable to communicatein accordance with a frequency hopping sequence; wherein themesh-networked devices are arranged to switch to a receive mode for afirst predetermined “wake-up” period and optionally remain active (i.e.in a receive or transmit mode) for a second predetermined “messageforwarding” period in a channel according to the sequence and switch toa receive mode for a third predetermined “acquisition-check” period andoptionally remain active (i.e. in a receive or transmit mode) for afourth predetermined “pairing” period in at least one otherpredetermined acquisition channel. The acquisition channel may be achannel specifically used to add or “pair” devices to the mesh-network.Thus, each time a mesh-networked device switches to an active mode, itmay be operable to first listen for any messages in the channelaccording to the frequency hopping sequence, and if a message isreceived, remain active to transfer it, and optionally forward on anacknowledgement; then, once the first predetermined “wake-up” period haspassed (and optionally the second predetermined “message forwarding”period, if a message is received, has passed) the device may be operableto switch to the acquisition channel for a third predetermined“acquisition-check” period.

The third predetermined “acquisition-check” period may be short, e.g.less than 0.1 s, less than 0.05 s, or less than 0.02 s, less than 1 ms,less than 0.5 ms, or less than 0.25 ms.

If no message is received on the acquisition channel during the thirdpredetermined “acquisition-check” period, the mesh-networked device mayreturn to the inactive mode. If a message is received from anunsynchronised device, it can remain active in the fourth predetermined“pairing” period and provide data to the unsynchronised device so thatthe unsynchronised device can join the mesh network.

The fourth predetermined “pairing” period may be less than one second,0.5 s, less than 0.2 s, or less than 0.05 s. The first predetermined“wake-up” period, the second predetermined “message forwarding” period,the third predetermined “acquisition-check” period and the fourthpredetermined “pairing” period may, in total be no more than 1 second,or no more than 0.5 seconds. This can allow synchronisation, or thetransmission of a message to the controller and back to occur in asingle channel. Of course, it is also possible for the synchronisationor transmission and acknowledgement to occur over a longer period inmore than one channel.

The mesh-networked devices may be operable to request to join thenetwork and to request data comprising the current channel in thefrequency hopping sequence and the time left in that channel.

The mesh-networked devices may be operable to transmit data (optionallyin response to a request) comprising the current channel in thefrequency hopping sequence and the time left in that channel to anunsynchronised device. This allows the unsynchronised device to join themesh-network and become a mesh-networked device as it can follow thefrequency hopping sequence and become time-synchronised with themesh-networked devices. It can also help synchronised devices remainin-sync.

An unsynchronised device may be a new device, e.g. a new sensor beingadded to an alarm network, or an existing device that has becomeunsynchronised and re-joins the network, for example, a battery powereddevice that has had its battery replaced.

Using an acquisition channel doubles the length of time thatmesh-networked devices remains active if no messages are received.However, this doubling is much less time than would be required if thedevices were to cycle through every channel in receive mode looking formessages from unsynchronised devices, or remain active long enough eachtime to pick up a preamble from an unsynchronised device cycling throughevery channel in transmit mode, to ensure a connection.

According to a second aspect of the invention, there is provided amesh-networkable device for use in a mesh network comprising acontroller and a plurality of mesh-networked devices, operable tocommunicate with the controller; mesh-networkable device comprising atiming unit, and being operable to communicate in accordance with afrequency hopping sequence; wherein the mesh-networkable device isoperable to switch between a transmit mode, in it is are capable oftransmitting messages to one or more other mesh-networked device and/orthe controller and an inactive mode, in which it is unable to transmitto or receive messages from one or more other mesh-networked devices orthe controller; wherein the mesh-networkable device is operable tosynchronise its timing unit to the mesh-networked devices (and/or thecontroller) and operable to switch to the transmit mode at pseudorandomtime intervals.

The mesh-networkable device according to the second aspect of theinvention may comprise any of the features of the first aspect of theinvention.

According to a third aspect of the invention, there is provided acontroller for controlling mesh-networked devices in a mesh network; thecontroller comprising a timing unit, and being operable to communicatein accordance with a frequency hopping sequence; wherein themesh-networked devices are operable to switch between a transmit mode,in which they are capable of transmitting messages to one or more othermesh-networked device and/or the controller and an inactive mode, inwhich they are unable to transmit to or receive messages from one ormore other mesh-networked devices or the controller; wherein thecontroller is operable to synchronise the timing units of themesh-networked devices to its timing unit such that the mesh-networkeddevices are operable to switch to the transmit mode at synchronisedpseudorandom time intervals.

The controller may be operable to transmit messages comprising dataincluding the length of time left in the channel in the frequencyhopping sequence.

The controller may be operable to transmit messages comprising dataincluding the current channel in the frequency hopping sequence.

The controller may comprise any of the features described in relation tothe first aspect of the invention.

According to a fourth aspect of the invention, there is provided amethod of operating a mesh-networked device to transmit data in a meshnetwork comprising a controller and a plurality of mesh-networkeddevices, operable to communicate with the controller; the methodcomprising communicating in accordance with a frequency hoppingsequence; whereby the mesh-networked device switches between a transmitmode, in which it is capable of transmitting messages to one or moreother mesh-networked device and/or the controller and an inactive mode,in which it is unable to transmit to or receive messages from one ormore other mesh-networked devices or the controller; the method furthercomprising synchronising a timing units of the mesh-networked devicewith timing units of other mesh-networked devices and/or the controllerand comprising switching to the transmit mode at pseudorandom timeintervals.

The method may comprise switching between the transmit mode, theinactive mode and a receive mode, in which the mesh networked device iscapable of receiving messages from one or more other mesh-networkeddevice and/or the controller. The method may comprise switching to thereceive mode in accordance with the frequency hopping sequence.

The pseudorandom time intervals may be, on average, at least 30 seconds,at least one minute, at least 2 minutes, at least 10 minutes or at least15 minutes.

The method may comprise transmitting at pseudorandom time intervalswhich are be synchronised to occur when other mesh network devices arein the receive mode. This may be achieved by calculating thepseudorandom time intervals such that they are always a multiple of thelength of time spent in each channel, such that they always end when thechannel changes (if the mesh-networked devices always receive when thechannel changes). Alternatively, the pseudorandom time interval need notbe a multiple of the length of time spent in each channel, but thedevice may wait for a pseudorandom time interval, then further delaytransmission until the other mesh networks are in a receive mode.

The method may comprise switching to the receive mode each time thefrequency channel changes according to the frequency hopping sequence.

The time in each channel in the frequency hopping sequence may beidentical, or the amount of time in each channel may be different fordifferent channels. Spending an identical time in each channel can bethe best use of resources, as the maximum time period can be spent ineach channel; over time, it also achieves a good spread across thefrequency spectrum. The time in each channel may be no more than 1second, no more than 0.5 seconds, or no more than 0.25 seconds.

The time in each channel may be substantially shorter than the averagepseudorandom time interval, for example at least 10 times shorter, atleast 50 times shorter, at least 100 times shorter, or at least 200times shorter.

The method may comprise switching to the receive mode and remaining in areceive mode for a first predetermined “wake-up” period, and if nomessage is received within that period, returning to the inactive mode.

The first predetermined “wake-up” period may be short, e.g. less than0.1 s, less than 0.05 s, less than 0.02 s, less than 1 ms, less than 0.5ms, or less than 0.25 ms.

The method may comprise switching to the receive mode and remaining inthe receive mode for a first predetermined “wake-up” period, and if amessage is received within that period, remaining in an active (receive,or transmit) mode for a second predetermined “message forwarding”period.

The second predetermined “message forwarding” period may be longer thanthe first predetermined “wake-up” period. The second predetermined“message forwarding” period may be sufficiently long to receive and thenforward a message. The method may comprise forwarding messages from onemesh-networked device and/or the controller to another mesh-networkeddevice and/or the controller where the messages contain instructions todo so. The second predetermined “message forwarding” period may besufficiently long to receive and forward a message to another device,and to receive and forward an acknowledgement from said other device.The method may comprise receiving and forwarding a message to anotherdevice, and to receive and forward an acknowledgement from said otherdevice.

The second predetermined “message forwarding” period may be less than0.5 s, less than 0.2 s, or less than 0.05 s. The first predetermined“wake-up” period and the second predetermined “message forwarding”period may, in total be no more than 0.5 seconds, or no more than onesecond.

The method may comprise waking up, listening briefly on the channel onwhich any transmissions would be received, and if no such transmissionis received, returning to the (low-power) inactive mode.

The method may comprise cycling through the channels in accordance withthe frequency hopping sequence, and may comprise cycling through thechannels consecutively, hopping to the next consecutive channel (up ordown) after a predetermined time interval.

The mesh-networked device may follow the same frequency hopping sequenceat the same time as other (optionally all other) devices in the meshnetwork.

The method may comprise synchronising the timing unit of themesh-networked device to a master clock, which may be provided in thecontroller.

The method may comprise sending acknowledgement messages comprising thecurrent frequency channel and time left in the channel.

The method may comprise receiving one or more acknowledgement messagescomprising the current frequency channel of a sending device and thetime left in that channel, compare that to the time left in the currentchannel to detect lack of synchrony with the sending device. The methodmay comprise correcting an internal timing unit and/or the timing oftheir frequency hopping based on the comparison with data in theacknowledgement message.

The method may comprise switching to the transmit mode according to apseudorandom time sequence.

The method may comprise switching to the transmit mode according to thesame pseudorandom time sequence as other mesh network devices in themesh network.

The method may comprise additionally switching to the transmit mode wheninstructed to forward a message by another mesh-networked device, or thecontroller. This in effect is still pseudorandom, since the instructionsto forward a message will be based on a pseudorandom transmission fromthe other mesh networked device or the controller.

The mesh-networked device may be battery powered.

The mesh-network device may be an alarm device

The mesh-networked device may comprise one or more sensors.

The method may comprise only sending data packets short enough to fitwithin a single channel, e.g. less than 0.5 s, less than 0.1 s, or lessthan 50 ms, when the mesh networked devices are synchronised.

The method may comprise switching to a receive mode for a firstpredetermined “wake-up” period and optionally remaining active (i.e. ina receive or transmit mode) for a second predetermined “messageforwarding” period in a channel according to the sequence and switchingto a receive mode for a third predetermined “acquisition-check” periodand optionally remaining active (i.e. in a receive or transmit mode) fora fourth predetermined “pairing” period in at least one otherpredetermined acquisition channel.

The acquisition channel may be a channel specifically used to adddevices to the mesh-network. Thus, the method may comprise, each time amesh-networked device switches to an active mode, first listening forany messages in the channel according to the frequency hopping sequence,and if a message is received, remaining active to transfer it, andoptionally forward on an acknowledgement; then, once the firstpredetermined “wake-up” period has passed (and optionally the secondpredetermined “message forwarding” period, if a message is received, haspassed) switching to the acquisition channel for a third predetermined“acquisition-check” period.

The third predetermined “acquisition-check” period may be short, e.g.less than 0.1 s, less than 0.05 s, or less than 0.02 s less than 1 ms,less than 0.5 ms, or less than 0.25 ms.

If no message is received on the acquisition channel during the thirdpredetermined “acquisition-check” period, the method may comprisereturning to the inactive mode. If a message is received from anunsynchronised device, the method may comprise remaining active in thefourth predetermined “pairing” period and providing data to theunsynchronised device so that the unsynchronised device can join themesh network.

The fourth predetermined “pairing” period may be less than 0.5 s, lessthan 0.2 s, or less than 0.05 s. The first predetermined “wake-up”period, the second predetermined “message forwarding” period, the thirdpredetermined “acquisition-check” period and the fourth predetermined“pairing” period may, in total be no more than 1 second or no more than0.5 seconds. This can allow synchronisation, or the transmission of amessage to the controller and back to occur in a single channel. Ofcourse, it is also possible for the synchronisation or transmission andacknowledgement to occur over a longer period in more than one channel.

The method may comprise sending a request to join the network andrequesting data comprising the current channel in the frequency hoppingsequence and the time left in that channel.

The method may comprise transmitting data (optionally in response to arequest) comprising the current channel in the frequency hoppingsequence and the time left in that channel to an unsynchronised device.

Obviously the mesh networked device of the second aspect of theinvention may be operable to carry out the method of the fourth aspectof the invention, including any optional features.

Indeed, any features defined herein may be combined with any otherfeatures, unless they are mutually exclusive, regardless of whether suchcombined features are defined in relation to still further features;that is to say, optional/preferred features can be combined withoutnecessarily also including the all the features of the statements ofinvention to which they refer—likewise, any features of the specificdescription may be combined in new claims.

DETAILED DESCRIPTION OF THE INVENTION

In order that the invention may be more clearly understood, embodimentsthereof will now be described, by way of example only, with reference tothe accompanying drawings, of which:

FIG. 1 is a schematic diagram of a mesh-network in accordance with theinvention;

FIG. 2 is a schematic diagram of a controller of the mesh network ofFIG. 1;

FIG. 3 is a schematic diagram of a mesh-networked device of FIG. 1;

FIG. 4 is a graph showing movement between frequency channels over timein the mesh network according to FIG. 1;

FIGS. 5a and 5b are graphs showing transmissions between devices in themesh network of FIG. 1;

FIG. 6 is another graph showing transmissions between devices and thecontroller in the mesh network of FIG. 1;

FIG. 7 is another graph showing transmissions between a device and thecontroller in the mesh network of FIG. 1; and

FIG. 8 is yet another graph showing transmissions between devices andthe controller in the mesh network of FIG. 1.

With reference to the figures, in particular FIG. 1, an exemplary meshnetwork 1 in accordance with the invention is shown. The mesh network 1is described in the context of an alarm system, but those skilled in theart will appreciate that the invention could be used in numerous otherapplications.

The mesh network 1 comprises a controller 2 and a series ofmesh-networked devices 3, or nodes. The mesh-networked devices 3 arearranged in a series of zones on the basis of the minimum number ofdevices required to forward a message from them to the controller 2.Thus, a zone 1 device 3 can communicate directly with the controller 2,a zone 2 device 3 can only communicate with the controller 2 via atleast one other (zone 1) mesh networked device, and a zone 3 device 3can only communicate with the controller 2 via at least two other meshnetworked devices 3 (one in zone 2 and the other in zone 1).

A mesh network 1 is a network topology in which each node relays datafor the network. All mesh-networked devices 3 cooperate in thedistribution of data in the network.

The mesh-network 1 of the invention uses a routing technique topropagate messages between the controller and the mesh-networkeddevices.

In this embodiment, messages are sent along the routes requiring thelowest number of re-transmissions—hence, in the figure each node 3 inzone 2 or 3 is connected to a node in a more inward zone (zone 1 or 2)with a full line, indicating a preferred route. Dotted lines are alsoshown, indicating where signal strength is sufficient for communicationwith other nodes, but routes along dotted lines will not be used unlessa preferred route fails and the mesh network self-heals.

The controller 2 includes a hub 4, which controls the mesh-networkeddevices 3, receiving periodic poll messages from them, to check thatthey are functional, arming them, and receiving messages from themconcerning changes in status, e.g. sensor data, which are of coursegenerated randomly.

The controller 2 also includes a data interface 5, through which itcommunicates with a security panel 6, computer 7, controllingapplication 8 (e.g. on a smartphone or the like), in order tocommunicate with a user, e.g. so as to receive arm/disarm signals viaany of the security panel 6, computer 7, controlling application 8 (e.g.on a smartphone or the like), and to send alerts to a user via the samedevices, or via the devices to other parties (such as through thesecurity panel to the police force, or a private security company, or toan external sounder, not shown).

As shown in FIG. 2, the hub 4 of the controller 2 comprises a powersupply 10, typically an external power supply, with in-built backupbatteries/capacitors, a microcontroller 11, programmed to processsignals received and sent by the controller 2, and buffer circuitry 12,through which the hub 4 outputs signals to, and receives signals fromthe data interface 5.

Additionally, the hub 4 comprises a transceiver 13, through which (viaantenna 14) the controller sends and receives signals so as to exchangemessages with the mesh-networked devices 3. The microcontroller 11 isalso connected to a timing unit 15, which includes a watch crystal, soas to maintain a master clock.

Like the controller 2, the mesh-networked devices 3, shown in FIG. 3,comprise a microcontroller 16 to process signals sent and received bythe mesh-networked device, a timing unit 17 including a watch crystal,in order to independently maintain time, and a transceiver 18 throughwhich (via antenna 19) the mesh-networked devices 3 send and receivesignals so as to exchange messages with other mesh-networked devices 3and/or the controller 2. Like the controller 2, the mesh-networkeddevices 3 have buffer circuitry 20, but in this instance, the buffercircuitry is provided to interpret signals received from a sensor 21,such as a light sensor, door sensor, or the like.

The mesh-networked devices 3 of this embodiment of the invention arebattery-powered wireless devices, hence, whereas the controller 2 isprovided with a battery-backed up external power supply, themesh-networked devices 3 comprise batteries 22. Provision of batterypower, rather than external power, makes setting up a mesh-network 1(i.e. installing the mesh-networked devices 3) much morestraightforward, but means that low power consumption is highlydesirable.

The mesh-network 1 of the invention is arranged to communicate using afrequency hopping spread spectrum (FHSS) communication technique.Accordingly, the microcontrollers 11, 16 of the controller 2 and themesh-networked devices 3 are programmed to switch between differentchannels according to a sequence which is the same for every meshnetworked device 3 in the mesh network 1 and for the controller 2. Thedevices 3 and the controller 2 switch between channels after apredetermined time interval (which may for example be the maximum periodpermitted in a single channel according to e.g. FCC regulations).

The frequency hopping sequence may be cyclic. For the sake ofexplanation, the frequency hopping sequence in the embodiment of theinvention is cyclic and consecutive, hopping to the next consecutivechannel up after a predetermined time interval.

FIG. 4 shows a first example of an arrangement of the mesh-network 1 ofthe invention, in which each mesh-networked device 3 and the controllerare operable to switch between 10 frequency channels. The example shownswitches cycles through every channel from Ch0 to Ch9 spending 0.5seconds in each channel and returning to channel Ch0 after 5 seconds, torepeat the cycle.

It is more typical to use a larger number of channels, thus, a secondexample described with reference to FIG. 5 relates to a 50-channelsystem. In this system, again, it can be seen that each mesh-networkeddevice, and the controller cycles through each channel starting atchannel Ch0 at time T0. The timing units 17 of all the mesh-networkeddevices 3 are synchronised to the same master clock held in the timingunit 15 of the controller 2, so time T0 is the same for all devices, andthe devices all cycle through, channels consecutively, spending 0.5seconds in each channel and therefore returning to channel Ch0 after 25seconds, 50 seconds, 75 seconds and so on. Of course, those skilled inthe art will appreciate that the microcontrollers could incorporateintegrated timing units, which could keep time on the basis ofassociated watch crystals.

FIG. 5a illustrates a first message transmission being sent from a firstmesh-networked device 3 after 10 seconds, which is therefore sent onchannel Ch20, and will therefore be received by another mesh-networkeddevice 3 and/or the controller 2 since it will also have switched tochannel Ch20 to receive (since 10 seconds have passed since T0 and itwill have incremented one channel every 0.5 seconds). After apseudorandom time interval, a second data packet transmission is sentfrom the first mesh networked device 3. In this example, thepseudorandom time interval is 137.5 seconds later (at 147.5 seconds fromT0), so the devices have completed 5 complete cycles through all 50channels (over the space of 125 seconds) and have incremented a further25 channels over the additional 12.5 seconds.

To those outside the system, the choice of channel Ch45 for the secondmessage appears random and unpredictable, since without any knowledge ofthe frequency hopping sequence (in terms of either the order in whichthe channels are hopped through, or the duration in each channel), it isimpossible to tell why after 137.5 seconds the transmission is onchannel Ch45. It is likewise impossible to tell why the secondtransmission occurred after 137.5 seconds.

The provision of a frequency hopping cycle that is completed over a muchshorter period than the pseudorandom time interval also makes itdifficult, if not impossible for a hacker, to work out the cycle andfollow it.

As shown in FIG. 5b , in one example, the pseudorandom time interval hasa fixed component A having a period in this particular example of 125seconds and a pseudorandom variable component B, in this embodiment of+/−20 seconds. The result is the same as a pseudorandom time intervalhaving only a pseudorandom component, but with that component having alower threshold of 105 seconds and an upper threshold of 145 seconds.

In practice, the fixed component A, or lower threshold should be set ashigh as possible for minimum power usage, and that time period willdepend on the particular usage. In an alarm system, a time period of upto 15 minutes between polls is conceivable. If the pseudorandomcomponent B (or the upper threshold minus the lower threshold) is set aslow as possible whilst being at least equal to the length of a completecycle of the sequence (25 seconds in a 50-channel system which spends0.5 seconds in each channel), to a hacker, the system may even appear tobe regular, but with an error margin (of +/−12.5 s in this example).

FIG. 6 illustrates the successful transmission of a message inaccordance with the second example discussed above. The message is aninformation-message containing information, for example a status-update,e.g. indicating that the device is armed, from a zone 3 mesh-networkdevice 3 to the controller 2; FIG. 6 also illustrates the return of anacknowledgement message. The message will include the address, oranother identifier for the originating device 3, so that the controllerknows which device sent it and has an address to send anacknowledgement.

Whilst all the devices 3 of this embodiment have the same construction,in this example, a device in zone 1 is indicated 3 a, a device in zone 2is indicated 3 b and a device in zone 3 is indicated 3 c.

In this example, a pseudorandom time after the last message wassent/received, but immediately after switching to a new channel, a zone3 device 3 c turns to an active state and sends an information messageto a zone 2 mesh-networked device 3 b in channel Ch16, that being thechannel dictated by the frequency hopping sequence at that point intime. Since all the timing units 15, 17 are synchronised, the zone 2mesh-networked device 3 b is also active at the moment that it switcheschannels and listening on channel Ch16 and receives the message from thezone 3 device 3 c. Having begun to receive a message in the firstpredetermined “wake-up” period after switching on (i.e. having receivedthe preamble) the zone 2 device remains active for a secondpredetermined “message forwarding” period, until it has received thefull message. Each mesh network device 3 is operable to remain in thereceive mode only for a brief first predetermined “wake-up” period of0.5 ms if it does not receive a message, so the zone 2 mesh networkdevice 3 b does not immediately send on the message to the zone 1 meshnetwork device 3 a, because the message is about 50 ms (i.e. 0.05s) longand the zone 1 device 3 a will have turned to the receive mode, receivednothing and returned to its inactive state by the time the message isreceived by the zone 2 device 3 b. Accordingly, the zone 2 device 3 bsends the message to the zone 1 device 3 a when they are both arrangedto next listen, in the next channel, i.e. in channel Ch17, half a secondafter they last woke up.

Having forwarded a message, the zone 3 device 3 c is operable to remainin the active state, in the receive mode when it becomes active in thenext channel Ch17, in order to await an acknowledgement. Similarly,after forwarding the message to the zone 1 device 3 a, the zone 2 device3 b remains in the receive mode, awaiting an acknowledgement.

Having received the message from the zone 2 device 3 b, the zone 1device 3 a forwards it to the controller 2. The controller 2 is alwaysactive, either in the transmit mode or the receive mode, since it has anexternal power source 10 and does not need to conserve energy.Consequently, the zone 1 device sends on the message, even though thefirst predetermined “wake-up” period (of 0.5 ms) has expired, andremains active to receive an acknowledgement.

The controller 2 processes the message and prepares an acknowledgementwith appropriate instructions. The acknowledgement message comprises anindication of the channel that the controller 2 is using and the timeremaining in that channel. The zone 1 device 3 a receives theacknowledgement message and immediately forwards it on to the zone 2device 3 b, also providing an indication of the channel that it (thezone 1 device 3 a) is in, and how long is left in that channel. The zone1 device 3 a compares the time that the controller 2 has left in thechannel with the time it has left in the channel and synchronises itsclock 17 if necessary (i.e. if the times do not match, or are outside anerror margin, e.g. 2 ms). It then switches to the inactive state.

The zone 2 device 3 b has remained active, so it receives theacknowledgement message on channel Ch17 and immediately forwards it onto the zone 3 device 3 c, also adding to the message an indication ofthe channel that it (the zone 2 device 3 b) is in and the length of timeleft in that channel Ch17. The zone 2 device 3 b compares the time thatthe zone 1 device 3 a and/or the controller 2 has left in its channelCh17 with the time it has left in the channel Ch 17 and synchronises itstiming unit 17 if necessary (i.e. if the times do not match, or areoutside an error margin, e.g. 2 ms). It then switches to the inactivestate.

The zone 3 device 3 c has remained active, so it receives theacknowledgement message on channel Ch17 and takes any necessary actionas dictated in the message from the controller 2. It also compares thetime that the zone 2 device 3 b and/or the controller 2 has left in itschannel with the time it has left in the channel and synchronises itstiming unit 17 if necessary (i.e. if the times do not match, or areoutside an error margin, e.g. 2 ms). It then switches to the inactivestate.

A third example of a manner of operation of the mesh network 1 inaccordance with the invention is described with reference to FIG. 7. Inthis example, in addition to sending messages in accordance with themethod set out in the second example, whereby channels are cycledthrough and a receive mode entered in a new channel every 0.5 s andmessages transmitted at pseudorandom time intervals, each mesh-networkeddevice 3, and the controller 2 listens on an acquisition channel, Ch0for any unsynchronised mesh-network devices 3 which wish to join themesh network 1.

FIG. 7 shows only a controller 2 and a single device 3 within range ofthe controller 2.

In this example, the controller 2 is operated to listen (i.e. toreceive) on channel Ch0 every 0.05 s, as it is externally powered. Onthe other hand, the mesh networked devices 3, only listen on channel Ch0each time they become active and enter the receive mode (each time theychange channel according to the frequency hopping sequence), andtransmit on channel Ch0 whenever they are unsynchronised; transmitting amessage with a request to join, in order to join the mesh-network 1. Inthis example, the controller is at channel Ch35 in the cycle through thefrequency hopping sequence, when it receives a signal on channel Ch0.

In this example, the unsynchronised mesh-networkable device 3 isswitched on at 0.175 s and immediately sends a request on channel Ch0 tojoin the network, then remains in receive mode on channel Ch0 awaiting aresponse. This request is received the next time the controller 2 is setto receive on channel Ch0, i.e. at 0.2 s. Having received the joinrequest from mesh-networkable device 3, the controller 2 verifies thatthe mesh-networkable device 3 is a mesh-networkable device intended forthe system, by comparing an ID number, for example, and having verifiedthe mesh-networkable device 3, the controller 2 sends an acknowledgementmessage on channel Ch0 which once again comprises an indication of thechannel that is being used (Ch35) and the length of time left in thatchannel.

On receipt of that acknowledgement message the mesh-networkable device 3configures itself, setting the timing unit 17 according to the time leftin the channel and setting the position in the frequency hoppingsequence stored in the device 3 to the channel Ch35 indicated by thecontroller 2. The mesh-networked device 3 then sends a confirmationmessage on the correct channel 35 in the frequency hopping sequenceconfirming that it is synchronised in time and frequency.

Now that the mesh-networked device 3 is synchronised in time andfrequency, it will follow the frequency hopping sequence, and thereforebe on the same channel at the same time as all the other mesh-networkeddevices 3 in the mesh-network 1 (which follow the same frequency hoppingsequence), in this example, hopping to (and entering the receive modeon) Channel Ch36 at T=0.5 s, to Channel 37 at T=1 s and so on. Inconsequence, it can exchange messages with other mesh-networked devices3 that are within range, as well as with the controller 2, and istherefore integrated into the mesh network 1.

Another example of the way the mesh network 1 operates to synchronisemesh-networked devices 3 is described with reference to FIG. 8. In thisexample, a device 3 b in zone 2 has become unsynchronised, for examplebecause its battery 22 has been replaced (the same technique would workfor a new device 3 joining in zone 2). Again in this example, the meshnetwork 1 (including the controller 2 and all mesh-networked devices 3)are operable to use an acquisition channel Ch0, in addition to thechannels Ch 1-49, used for communication at pseudorandom intervals.

The mesh networked zone 1 device (3 a) and the controller 2 are workingas normal, with the mesh-networked device 3 a switching channels insequence, entering the receive mode briefly and switching to an inactivestate when no message is received; the mesh networked device 3 a, asalluded to earlier, also listens on the acquisition channel Ch0 eachtime it is active, after listening on the predetermined channel in thesequence and before becoming inactive. In this example, at T0, themesh-networked zone 1 device 3 a and the controller 2 are in channelCh26. The graph starts shortly before the device 3 b in zone 2 is turnedon, so having heard nothing in either Ch0 or Ch26, the device 3 abecomes inactive after 1 ms (0.5 ms in Ch26 and 0.5 ms in Ch0). Thecontroller remains active, switching frequently between Ch26 and Ch0listening for messages.

When the unsynchronised mesh networkable device 3 b is switched on (atT=0.2 s), it immediately transmits on Ch0, sending a request to join thenetwork 1. As the device is outside the range of the controller 2, thismessage is not received by the controller 2, although it is listening onCh0. Thus, it is only at T=0.5005 s when the Zone 1 device 3 a has wokenup, listened in channel Ch27 (the next in the sequence) and switched tothe acquisition channel Ch0, that the message from the unsynchronisedzone 2 device 3 b is received by the zone 1 device 3 a. Having receivedthe message, the zone 1 device, forwards it to the controller 2 onchannel Ch0 (of course, the mesh networked device 3 a could instead usethe channel Ch27 in the sequence, knowing that the controller will belistening in that channel (Ch27) too). The controller 2 verifies thatthe zone 2 device 3 b that has become unsynchronised is amesh-networkable device 3 according to the system and sends anacknowledgment on Channel Ch0 destined for the zone 2 device 3 b(addressed to it) comprising the details of the current channel Ch27 andthe amount of time left in the channel Ch27. The zone 1 device 3 a,receives the acknowledgement and forwards it on to the zone 2 device 3b. The zone 2 device 3 b is therefore able to update its internal clock17 and become synchronised to the mesh network 1. Now that the zone 2device 3 b has become a synchronised mesh-network device 3, it is ableto communicate with the other devices 3 in the mesh network 1, to followthe sequence, receiving communications from other devices 3 on thecorrect channel in the sequence at the correct time, and transmittingmessages (for example including sensor data or status data) atpseudorandom intervals on the correct channel.

Of course, the discussions above are generally concerned withtransmission of non-time-sensitive polling-type messages, in which nodes“check-in”. However, especially in the context of a mesh-networked alarmsystem, it is to be expected that additionally to the messages which arenot time-sensitive and are sent at pseudorandom intervals, there will befurther alarm messages, generated at random times (when a sensor istriggered), which will of course be forwarded straight away, withoutwaiting for a predetermined pseudorandom time interval. Instead, alarmmessages will be sent in the appropriate channel at the time when themesh network devices wake up. For example, if the channel is say CH30and the first predetermined “wake-up” period for receiving messages hasexpired in that channel, the alarm message will be sent in the nextchannel in the sequence, CH31, during the next first predetermined“wake-up” period, when each mesh networked device 3 turns to an activestate to receive messages.

Indeed, those skilled in the art may envisage other situations where atime-sensitive message must be sent as soon as possible (i.e. at thenext moment that it will be received), and it is envisaged that theinvention, (also including certain messages which are only sent atpseudorandom time intervals), will be included in such systems.

The above embodiments are described by way of example only. Manyvariations are possible without departing from the scope of theinvention.

The invention claimed is:
 1. A mesh network comprising a controller anda plurality of mesh-networked devices, operable to communicate with thecontroller; the controller and the plurality of mesh-networked devicescomprising timing units, and being operable to communicate in accordancewith a frequency hopping sequence; wherein the mesh-networked devicesare operable to switch between a transmit mode, in which they arecapable of transmitting messages to one or more other mesh-networkeddevice and/or the controller and an inactive mode, in which they areunable to transmit to or receive messages from one or more othermesh-networked devices or the controller; wherein the timing units ofthe mesh-networked devices are synchronised and the mesh-networkeddevices are operable to switch to the transmit mode at time intervalshaving a pseudorandom component.
 2. A mesh network according to claim 1wherein the mesh-networked devices are operable to switch between thetransmit mode, the inactive mode and a receive mode, in which they arecapable of receiving messages from one or more other mesh-networkeddevice and/or the controller.
 3. A mesh network according to claim 2wherein the mesh-networked devices are operable to switch to the receivemode in accordance with the frequency hopping sequence.
 4. A meshnetwork according to claim 2 wherein the time intervals having apseudorandom component are synchronised to occur when other mesh networkdevices are in the receive mode.
 5. A mesh network according to claim 2wherein the mesh-networked devices switch to the receive mode each timethey change frequency channel.
 6. A mesh network according to claim 2wherein the plurality of mesh-networked devices are operable to switchto the receive mode and remain in a receive mode for a firstpredetermined “wake-up” period, and if no message is received withinthat period, to return to the inactive mode.
 7. A mesh network accordingto claim 1 wherein the time in each channel in the frequency hoppingsequence is identical.
 8. A mesh network according to claim 7 whereinthe frequency hopping sequence is cyclic.
 9. A mesh network according toclaim 8 wherein the frequency hopping sequence is cyclic andconsecutive.
 10. A mesh network according to claim 1 wherein the time ineach channel in the frequency hopping sequence is at least 10 timesshorter, at least 50 times shorter, at least 100 times shorter, or atleast 200 times shorter than an average time interval having apseudorandom component.
 11. A mesh network according to claim 1 whereinthe mesh-networked devices are operable to send acknowledgement messagescomprising the current frequency channel and time left in the channel.12. A mesh network according to claim 1 wherein the mesh-networkeddevices are battery powered.
 13. A mesh network according to claim 1wherein the plurality of mesh-networked devices, are operable tocommunicate in accordance with a frequency hopping sequence; wherein themesh-networked devices are arranged to switch to a receive mode for afirst predetermined “wake-up” period and remain active in a receiveand/or transmit mode for a second predetermined “message forwarding”period in a channel according to the sequence and switch to a receivemode for a third predetermined “acquisition-check” period and remainactive in a receive and/or transmit mode for a fourth predetermined“pairing” period in at least one other predetermined acquisitionchannel; wherein the acquisition channel is a channel specifically usedto add devices to the mesh network and each time a mesh-networked deviceswitches to an active mode, each mesh-networked device is operable tofirst listen for any messages in the channel according to the frequencyhopping sequence, and if a message is received, remain active totransfer it, and forward on an acknowledgement; then, once the firstpredetermined “wake-up” period has passed and the second predetermined“message forwarding” period, if a message is received, has passed thedevice is operable to switch to the acquisition channel for a thirdpredetermined “acquisition-check” period.
 14. A mesh-networkable devicefor use in a mesh network comprising a controller and a plurality ofmesh-networked devices, operable to communicate with the controller;mesh-networkable device comprising a timing unit, and being operable tocommunicate in accordance with a frequency hopping sequence; wherein themesh-networkable device is operable to switch between a transmit mode,in it is are capable of transmitting messages to one or more othermesh-networked device and/or the controller and an inactive mode, inwhich it is unable to transmit to or receive messages from one or moreother mesh-networked devices or the controller; wherein themesh-networkable device is operable to synchronise its timing unit tothe mesh-networked devices and/or the controller and operable to switchto the transmit mode at time intervals having a pseudorandom component.15. A controller for controlling mesh-networked devices in a meshnetwork; the controller comprising a timing unit, and being operable tocommunicate in accordance with a frequency hopping sequence; wherein themesh-networked devices are operable to switch between a transmit mode,in which they are capable of transmitting messages to one or more othermesh-networked device and/or the controller and an inactive mode, inwhich they are unable to transmit to or receive messages from one ormore other mesh-networked devices or the controller; wherein thecontroller is operable to synchronise the timing units of themesh-networked devices to its timing unit such that the mesh-networkeddevices are operable to switch to the transmit mode at synchronised timeintervals having a pseudorandom component.
 16. A controller according toclaim 15 which is operable to transmit messages comprising dataincluding the length of time left in the channel in the frequencyhopping sequence.
 17. A method of operating a mesh-networked device totransmit data in a mesh network comprising a controller and a pluralityof mesh-networked devices, operable to communicate with the controller;the method comprising communicating in accordance with a frequencyhopping sequence; whereby the mesh-networked device switches between atransmit mode, in which it is capable of transmitting messages to one ormore other mesh-networked device and/or the controller and an inactivemode, in which it is unable to transmit to or receive messages from oneor more other mesh-networked devices or the controller; the methodfurther comprising synchronising a timing units of the mesh-networkeddevice with timing units of other mesh-networked devices and/or thecontroller and comprising switching to the transmit mode at timeintervals having a pseudorandom component.
 18. A method according toclaim 17 comprising switching between the transmit mode, the inactivemode and a receive mode, in which the mesh-networked device is capableof receiving messages from one or more other mesh-networked devicesand/or the controller; and further comprising switching to the receivemode in accordance with the frequency hopping sequence.
 19. A methodaccording to claim 17 comprising transmitting at time intervals having apseudorandom component which are be synchronised to occur when othermesh network devices are in the receive mode.
 20. A method according toclaim 17 comprising calculating the time intervals having a pseudorandomcomponent such that they are always a multiple of the length of timespent in each channel, such that they always end when the channelchanges.